Front wipers won't work correctly when shut off. what is the cause of this?

Thsi is for a 2002 Mercury Villager:
It sounds harder than it is: 4mm Allen wrench and 10 min.
Warning: The wiper arm/ motor assembly MOVES in a circular motion. Use care when exposing this assembly.
If your wipers don't park (shut off in the up position) or if they stay in the up position when in the delay mode, you probably have to reset the cam pVosition on the arm assembly. Remove the plastic cap covering the arm / motor assembly located under a plastic cover on the passenger firewall. Once you remove the plastic "hinged" cap on the arm/motor assembly, you will see a silver cam. Note that there is a pin that seats in a groove on the cam. If you had shut off the wipers, and they didn't park, look at the pin location. It should be at least 90' clockwise of the groove. In order to adjust this you will need a 4mm allen wrench to remove the center screw so you can lift the arm assembly off the motor. Lift the motor up and move the wipers by hand to the park position. Turn the cam so the pin moves almost 180' from the seating groove (you may have to partially place the assembly back slightly on the mounting post)this will allow the cam/arm to be placed back on the motor and have enough length in the arm to seat it. (When the top part of the cam is turned, it moves in an elliptical motion and therefore has a "short" and "long" setting.) Place the arm/cam assembly back on the post making sure the bottom of the cam, which has a cut out in the bottom and you will also see a little spring/sensor that is mounted on the underside. This spring/sensor has to be 4-5 mm away from the tab on the mounting surface. Seat the assembly with the cutout portion of the cam centered with the tab on the mounting surface. This will get you real close to that 4-5mm distance. Tighten allen screw to 125 inch pounds.
The Only time the pin is in the groove is when it is DELAY mode.
If you have the pin in the groove when it is parked, it is wrong and more than likely the wipers are not down all the way or the wipers stay in the up position when in the delay mode before cycling. Hope this helps. Posted on Jan 10, 2016

Check the wiper assembly, on the wiper arm has nut I guess it is 17mm .take off the small cover and then loose the nut,pull out the wiper arm while it is in upright position put back the wiper arm to the desired position and tighten the nut . hope it helps.

The wiper park switch in the wiper motor is defective is my guess.